The present invention relates to a system and method to control engine idle speed of an internal combustion engine coupled to an automatic transmission with a torque converter, and more specifically to controlling the engine idle speed in a drive range of the automatic transmission.
Engine idle speed control systems for an internal combustion engine of a vehicle are adapted to control an amount of air flow which is introduced to the engine (hereinafter referred to as an idle air flow amount), so as to match engine speed with target idle speed during an idle operation of the engine.
Japanese Patent Application First Publication No. 2000-45834 discloses an engine idle speed control system in which when an automatic transmission is operated in a drive (D) range during engine idle operation at a stop state of the vehicle, a basic idle air flow amount is corrected to increase based on a D-range idle-up correction value and idle speed feedback control is conducted to control the idle air flow amount such that engine speed is matched with a target idle speed. When the vehicle starts at D range state of the automatic transmission, the feedback control is stopped and the increased basic idle air flow amount is corrected by subtracting a vehicle speed correction value which is determined based on vehicle speed therefrom. This related art aims to prevent excessive increase in the idle air flow amount during the vehicle traveling.
Further, there has been proposed an engine idle speed control system in which when the vehicle speed exceeds a set value, for example, 4–6 km/h, the feedback control is prohibited and the idle air flow amount is controlled to a constant value, while when the vehicle speed is not more than the set value, the feedback control is permitted. Recently, there is a demand for facilitating transition to the feedback control by enhancing the set value of the feedback permission vehicle speed (feedback prohibition vehicle speed), thereby enhancing convergence of idle speed to a target idle speed and improving fuel economy.